1. Field of Invention
This invention relates to a method of preparing L- and D-5,5,5,5xe2x80x2,5xe2x80x2,5xe2x80x2-hexafluoroleucine and protected analogs thereof, including compounds having a protected amino functional group, compounds having a protected carboxy functional group, and compounds having protected amino and carboxy functional groups. These compounds are useful for the preparation of fluorous peptides and proteins.
2. Discussion
Hexafluoroleucine is of considerable interest as an extensively fluorinated analog of leucine that may be used as a building block in the design of fluorous proteins and peptides (E. N. G. Marsh, 7 Chem. Biol. R153 (2000)). Fluorocarbons have long been known for their chemical inertness, and their unique physicochemical properties have found industrial and medical uses as fire retardants, refrigerants, anesthetics and biologically inert polymers. The tendency of extensively fluorinated, or fluorous, organic molecules to partition into perfluorinated solvents has been exploited in organic synthesis to facilitate purification of products from reaction mixtures (I. T. Horvath, 31 Acc. Chem. Res. 641 (1998); Z. Y. Luo et al., 291 Science 1766 (2001)).
Recently, there has been much interest in whether the novel properties exhibited by fluorocarbon polymers can be exploited in the design of biological macromolecules. Extensively fluorinated analogs of hydrophobic amino acids, when substituted into proteins and peptides, pack into the hydrophobic core of the protein to produce proteins that combine novel physicochemical properties with biological activity (Y. Tang et al., 40 Biochemistry 2790 (2001)). Peptides designed to form dimeric coiled-coil structures based on the xe2x80x9cleucine zipperxe2x80x9d domain of the transcription factor GCN4, or de-novo designed sequences that incorporate (4R, 4S)-L-trifluoroleucine, (3R, 3S)-L-trifluorovaline or L-hexafluoroleucine, display increased stability, enhanced self association and stronger receptor-ligand binding than their non-fluorinated counterparts.
Hexafluoroleucine (hFLeu) is a highly fluorinated analog of leucine, an amino acid that plays an important role in the folding of many proteins. A racemic synthesis of hexafluoroleucine was reported in 1968, and the first chiral synthesis of this amino acid was reported in 1998 (J. Lazar and W. A. Sheppard, 11 J. Med. Chem. 138 (1968); C. Zhang et al., 81 Helv. Chem. Acta 174 (1998)). The highest enantiomeric purity achieved in this prior work was 81% e.e. and this weakness substantially limits the value of the route for the production of hFLeu for peptide synthesis. More recently, Kumar and coworkers (X. Xing et al., 3 Org. Lett. 1285 (2001)) reported a ten-step synthesis of L-hFLeu from D-serine. This synthesis was reported to provide L-hFLeu in  greater than 51% yield and  greater than 99% e.e. from Gamer""s aldehyde, which in turn is prepared from D-Serine in five steps. This synthesis is long, difficult to reproduce, and provides material whose enantiomeric purity cannot be demonstrated using the method described. In addition, Garner""s aldehyde is an expensive starting material, currently costing about $100 per g.
The present invention provides a comparatively short and efficient method for preparing L-5,5,5,5xe2x80x2,5xe2x80x2,5xe2x80x2-hexafluoroleucine (L-hFLeu) and its protected analogs from commercially available and inexpensive starting materials, including N-Cbz-L-serine. The claimed method can also be used to prepare D-hFLeu and its protected analogs by utilizing starting materials having opposite stereochemistry (e.g., N-Cbz-D-serine). Hexafluoroleucine and its protected analogs are useful for preparing fluorous peptides and proteins having known utility.
One aspect of the present invention provides a method of making a compound represented by Formula I, 
or a corresponding stereoisomer having opposite stereochemistry of Formula I, which can be represented by Formula Ixe2x80x2, 
In Formula I and Formula Ixe2x80x2, R1 and R2 are, respectively, N-terminal and C-terminal protecting groups. The method includes providing a compound having a tertiary hydroxy group as represented by Formula IV, 
or providing a corresponding stereoisomer having opposite stereochemistry of Formula IV, and displacing the tertiary hydroxy group to yield the compound of Formula I or its corresponding stereoisomer (Formula Ixe2x80x2). The method optionally includes de-protecting the compound of Formula I or its corresponding stereoisomer by replacing R1 or R2 or both R1 and R2 with a hydrogen atom.
Another aspect of the present invention provides a method of making the compound of Formula IV or its corresponding stereoisomer. The method includes reacting a compound of Formula III, 
or a corresponding stereoisomer having opposite stereochemistry of Formula III, with zinc to form an organozinc reagent, and subsequently reacting the organozinc reagent with hexafluoroacetone to yield the compound of Formula IV or its corresponding stereoisomer.
A further aspect of the present invention provides a method of making the compound of Formula I or its corresponding stereoisomer (Formula Ixe2x80x2), which includes reacting a compound of Formula II, 
or a corresponding stereoisomer having opposite stereochemistry of Formula II, with an iodinating agent to yield the compound of Formula III or its corresponding stereoisomer. The iodide is subsequently reacted with zinc to form an organozinc reagent, which in turn is reacted with hexafluoroacetone to form the compound of Formula IV or its corresponding stereoisomer. The compound of Formula IV and its corresponding stereoisomer have a tertiary hydroxy group, which is displaced using radical deoxygenation to yield the compound of Formula I or its corresponding stereoisomer. The method optionally includes de-protecting the compound of Formula I or its stereoisomer by replacing R1 or R2 or both R1 and R2 with a hydrogen atom.
An additional aspect of the presented invention includes synthetic intermediates, which can be used to prepare the compounds of Formula I and Formula Ixe2x80x2. These intermediates include compounds having structures represented by Formula IV or stereoisomers having opposite stereochemistry of Formula IV.